ReferenceError on lazy

I'm trying to add this simple image lazyload in my wordpress site because looks better and smaller than other js scripts but Im getting Uncaught ReferenceError: _ is not defined at window.addEventListener('scroll', _.throttle(lazyLoad, 16));

The full js code:

let lazyImages = [...document.querySelectorAll('.lazy')];
let inAdvance = 300;

function lazyLoad() {
  lazyImages.forEach(image => {
    if (image.offsetTop < window.innerHeight + window.pageYOffset + inAdvance) {
      image.src = image.dataset.src;
      image.onload = () => image.classList.add('loaded');
    }
  });

  // if all loaded removeEventListener
}

lazyLoad();

window.addEventListener('scroll', _.throttle(lazyLoad, 16));
window.addEventListener('resize', _.throttle(lazyLoad, 16));